U.S. Patent Application Publication No. 2003/0189211 A1 discloses an LED arrangement and an LED operating method having an automatic background illumination for portable electronic devices, for example, cell telephones, which manage without a separate light sensor, in that a microprocessor connected to the LED alternately causes the LED to illuminate and to operate as a light sensor. By rapidly switching over between the two operating modes, the LED appears to illuminate continuously. The brightness of the background illumination can be adapted automatically to the measured ambient luminosity.
Pulse width modulation (PWM) is frequently used to control the brightness of LEDs (light-emitting diodes), wherein the duty cycle of a square-wave signal supplied to the LED, i.e., the ratio of the pulse duration to the period duration, is varied.
PWM-controlled light-emitting diodes are also used in adaptive cockpit illumination in motor vehicles and/or the background lighting thereof. The ambient luminosity is typically measured by means of a separate light sensor, for example, a photodiode, to regulate the cockpit illumination level or background illumination level accordingly.
Such a separate light sensor, its installation, and its wiring are linked to a certain additional expenditure. The light sensor can be influenced by the light of the LEDs, and it can be shaded, for example, by dirt. In addition, the light measurement direction is possibly not optimal.
The circuit disclosed in U.S. Patent Application Publication No. 2003/0189211 A1 does manage without a separate light sensor, but it presumes that a microprocessor is present. However, this microprocessor should not be very remote from the LED, because the photocurrents driven by LEDs are very small. Corresponding conditions are often not readily provided, for example, in the case of cockpit illumination.
It would be desirable to provide an LED arrangement and a control method, which enable ambient-light-dependent brightness control with particularly little expenditure, in that they require neither a separate light sensor nor a microprocessor.